What an ADU Costs in Utah

Search for ADU prices and you will find figures that range so widely they are nearly useless. That is not because builders are hiding something. It is because an ADU's cost is decided mostly by the property it sits on and the type of unit, and those vary more than almost anything else in residential construction. This guide explains where the money goes, which fees we could confirm from official sources in Salt Lake County, and what we need to know to estimate your project.

The biggest decision: convert or build new

The type of ADU sets the baseline more than any other factor.

• Internal and basement ADUs use a structure, roof and foundation that already exist. The costs go into making the space a separate, code-compliant dwelling: egress, fire separation, a kitchen and bathroom, ventilation and often a separate entrance. See basement ADU cost.
• Garage conversions also reuse a structure, but a garage was never built to be lived in. The slab, walls, insulation, windows and utilities usually all need work, and lost parking may have to be replaced. See garage conversion cost.
• Detached and attached new builds involve everything: foundation, framing, roof, exterior, systems and finishes, plus the site work and utility connections to reach them. See detached ADU cost.

Site and utility costs

These are the costs homeowners most often underestimate, and they have nothing to do with how nice the unit is.

• Sewer: a detached ADU needs a sewer connection, usually tied into the existing lateral. The distance, depth and slope of that run, and whether it has to cross a driveway or large trees, can move the budget significantly. Some lots need a pump.
• Water: a new line from the main house or the meter, sized for a second dwelling.
• Electrical: many existing panels do not have capacity for a second dwelling with its own kitchen and heating. A service upgrade involves the utility as well as the electrician.
• Gas: if used, another line and possibly a larger meter.
• Access and grading: a backyard that equipment cannot reach means more hand work. Slopes add retaining, drainage and engineering.

Several Salt Lake County cities, including Millcreek, Murray and South Salt Lake, prohibit separate utility meters for an ADU, so service is shared with the main house. That affects how systems are sized. Our guide to ADU utility connections goes into each one.

Structure and foundation

New detached units need a foundation designed for the soil, frost depth and seismic requirements that apply along the Wasatch Front. Units above a garage need the existing structure evaluated and often reinforced. Basement conversions may need a new egress opening cut through a foundation wall, with a window well. Each of these can require an engineer and adds to the permit drawings.

Size, layout and finishes

Size matters, but less directly than people expect. Kitchens and bathrooms are the most expensive rooms in any home, and every ADU has at least one of each, so a smaller ADU costs more per square foot than a larger one. That is why cost per square foot is a misleading way to compare ADU bids. Beyond that, the choices you control are real: cabinet and fixture level, flooring, window count and size, ceiling height and exterior materials. Some cities, Millcreek among them, require exterior materials and colors that relate to the main house, which should be part of the budget from the start.

Soft costs

These are the costs of getting to construction: design, engineering, surveys where needed, city application and review fees, building permit fees and utility district charges. They are a smaller share than construction, but they come first, which is why a feasibility review before full design is worth doing.

Official fees we could confirm

Fees change and depend on the project, so treat these as examples from official schedules, checked on the date shown in the sources below, not as a complete list:

Building permit fees are usually calculated from the project's valuation and are separate from these. Renting the ADU typically requires a city business license, which carries its own fee. State law bars impact fees on an internal ADU within an existing home (Utah Code 11-36a-202), but that exemption does not extend to detached ADUs.

What we need to estimate your ADU

A useful estimate starts with the property, not a price list. We look at:

1. The address and zone, so the right city's rules and fees apply.
2. The ADU type and rough size you have in mind, and who it is for.
3. The lot: where the unit could go, access, slope and trees.
4. Where the sewer lateral, water line and electrical service are, and the capacity of the panel.
5. For conversions, the condition of the existing space.
6. Your finish expectations and a budget range, even a rough one.

From that, the budget can be built in the right order: fixed site and utility costs first, then the unit, then the choices you control. Our ADU budget planning guide explains how to set a contingency and plan allowances.

Paying for it

Most homeowners use some combination of savings, a home equity loan or line of credit, or a cash-out refinance. Salt Lake City's Redevelopment Agency offers the Backyard Keys ADU Loan Program to eligible owner-occupants west of I-15. See how to finance an ADU. If rental income is part of the plan, read ADU rental income for the assumptions worth testing; we do not promise any rental return.

Questions about ADU costs